The invention relates to a method and apparatus for reducing ground current interference with electrical signals representative of seismic wave signals passing from at least one geophone to a recording system via an electric circuit. The electric circuit includes relatively long electric cables lying on the surface of the earth.
Ground currents, or alternating currents travelling through the surface of the earth interfere with the electrical signals in these circuits. Those currents may be generated by various sources, such as domestic power lines, power supply of electric railways, and the operation of equipment designed for cathodically protecting buried metal structures (e.g., pipelines).
When carrying out seismic surveys in areas where ground currents occur, part of these currents will pass through the leads of the electric circuits that electrically connect a group of geophones with the recording system, and interfere with the electrical signals passing through these circuits, these electrical signals being representative of the seismic wave signals detected by the geophones. The currents enter the circuits through the resistive leakages and capacitive couplings of the circuits (such as the electric cables forming part of these circuits) and of the geophone coils to ground.
Several techniques have been attempted in the past to alleviate this interference of the ground currents with the seismic signals, which leads to unreliable seismograms recorded by the recorder of the recording system. In one of these techniques, a balancing circuit is used, which circuit comprises two pairs of arms that are electrically connected between the ground and the leads of the electric cable that leads from a group of geophones to a recording system. Each pair of arms comprises at least one variable impedance, and by adjusting the impedances the bridge circuit including the geophone(s), the electric cable, the balancing circuit and at least part of the recording system can be balanced such that the influence of the ground currents on the recorded signal is substantially suppressed.
It will be appreciated that the balancing circuit should be situated close to the input of the recording system, the latter system including suitable amplifying means, filter means, encoding station means and one or more recorders. After the balanced situation has been reached by adjusting the impedances of the balancing circuit, a seismic shot is fired, whereafter the seismic waves are detected by the geophones after their return to the earth surface, and passed on in the form of electrical signals through the electric circuit to the recording system.
Adjustment of the balancing circuit for balancing purposes requires the manual control of two knobs. By turning one of the knobs, the impedance of a first arm of the circuit (such as an arm consisting of a variable resistor) is varied such that the influence of the ground currents on the recorder is made minimal. Subsequently, the second knob is adjusted thereby varying the impedance of a second arm (which second arm belongs to a pair of arms not comprising the first arm) in order to further reduce this influence to a minimal value. The knobs are alternately adjusted in order to reduce the interference of the ground currents to as small a value as possible.
It will be appreciated that although adjustment of a balancing circuit is a simple operation, the adjustment required to reduce the interference of ground currents with seismic signals that are detected by a plurality of geophones divided up in twenty-four, forty-eight or even ninety-six groups is a time-consuming operation, since the twenty-four, forty-eight or ninety-six electric pairs of leads extending between the groups and the recording system are each provided with a separate balancing circuit. In fact, adjustment of all these balancing circuits prior to shooting is almost impossible since the leakage-to-ground pattern is not constant, as a result whereof the first-balanced balancing circuit will already require rebalancing before the last of the other balancing circuits has been balanced.